1. Introduction
Zinc oxide (ZnO) is a typical II-VI-semiconductor with exploitable properties, such as low work function, piezoelectricity, biocompatibility, and (transparent) electrical conductivity, to name a few [
1,
2,
3,
4,
5,
6]. In recent years, zinc oxide (ZnO) nanostructures have gained major interest because many different morphologies can be synthesized offering a broad field of applications. According to the desired properties of ZnO nanostructures and the variety of achievable geometries, several applications including the use as chemical sensors, electronics, devices in photocatalysis and for medical purposes have been realized [
1,
2,
3,
4,
5,
7,
8,
9,
10]. Specifically, arrays of one-dimensional ZnO nanostructures are promising candidates for electron field emission applications due to their high aspect ratio and low work function of the material [
4,
5].
ZnO nanowires can be produced by chemical vapor deposition, solvo- and hydrothermal wet chemical synthesis and physical vapor deposition methods [
7,
11,
12,
13,
14,
15,
16,
17,
18,
19,
20]. The vapor-solid growth of ZnO nanowires using zinc acetylacetonate hydrate and oxygen as precursors was first reported in 1999 [
21]. The vaporization of zinc acetylacetonate hydrate occurs at much lower temperatures (75–135 °C) than the temperatures required for vaporizing Zn or ZnO powder (above 550 °C) [
22,
23]. Compared to previous vapor transport methods, this synthesis route enables the growth of ZnO nanowires at a comparably low temperature and without catalyst as needed for the vapor-liquid-solid growth mechanism of nanowires [
13,
24,
25,
26]. Although ZnO nanowire fabrication is possible at temperatures below 100 °C and without catalyst by hydrothermal synthesis, there are several advantages for using vapor-solid growth, such as a higher purity of the nanowires, a single step process with only two precursor compounds (zinc precursor powder and oxygen gas), and the formation of sharp tips which are beneficial for the electron field emission application [
2,
15,
22]. Up to now, several studies only explored the dependence of the nanowire’s morphology on individual vapor-solid growth parameters, namely growth temperature, substrate type, gas flow, used amount of zinc precursor or growth time [
21,
22,
23,
27,
28]. However, to tailor the morphology of the ZnO nanowires for specific applications, the influence of all growth parameters on the wires morphology has to be known.
The use of one-dimensional ZnO nanostructures synthesized by vapor transport methods for electron field emission—releasing an electron from the solid to vacuum upon applying a strong external electrical field—was previously reported [
13,
29,
30,
31,
32,
33,
34,
35,
36,
37,
38]. The nanowires in these studies were grown via the vapor-liquid-solid or vapor-solid (VS) mechanism (sketched in
Figure 1a). However, for ZnO nanowires grown via the vapor-solid (VS) mechanism only, ZnO powder (partially in combination with graphite as a catalyst) or Zn powder and oxygen were used as precursors at growth temperatures between 550 and 700 °C. Note, the use of zinc acetylacetonate hydrate and oxygen as a precursor source for field emitter devices grown by VS has not been reported, so far.
In this work, the low-temperature vapor-solid growth of ZnO nanowhiskers in a three-zone tube furnace by utilizing zinc acetylacetonate hydrate and oxygen as precursors were investigated in detail. A comprehensive study of ZnO nanowhisker array growth as a function of the process parameters, namely temperature (500–650 °C), substrate type and position, gas flow as well as zinc precursor amount and growth time, is reported. For the first time, electron field emission characteristics of ZnO nanowhisker array devices grown in optimized conditions with the precursor combination are investigated using a home-built electron field emission setup.
4. Field Emission Measurements
Characteristic field emission—the non-linear increase of emission current—is observed for n-doped Si, Ti film on Si, and a Ti-coated 1.5 µm-thick freestanding SiN membrane upon increasing the applied electrical field (
Figure 11a). In detail, the turn-on field—here defined as the applied electric field necessary for the emission current to overcome a 5σ threshold from the data mean—is found to be lower for ZnO nanowhiskers grown on n-doped Si than for nanowhiskers grown on Ti films. Note, the local electric field at the tip of an emitter can be in orders of magnitudes higher than the externally applied electric field, because of the geometrical field enhancement effect, which strongly depends on the emitters’ shape. Linearization of the experimental data according to the Fowler-Nordheim (FN) theory for electron field emission permits the extraction of physical properties of the emitter [
45]. By taking a work function of 5.3 eV for ZnO into account [
46], the field enhancement factors—describing the ratio between the macroscopic applied electric field and the actual electric field at the surface of the emitter—have been derived for the ZnO nanowhiskers grown on different substrate types (
Figure 11b). The field enhancement factor γ for ZnO nanowhiskers grown on n-doped Si and Ti film on bulk Si vary within their standard deviations, which were calculated from the linear fits. However, γ for ZnO nanowhiskers grown on a Ti-coated silicon nitride membrane is lower and will be considered as a separate case in further discussion.
Theoretical calculations based on FN-theory showed that a macroscopic electric field of more than 1700 V/µm would be necessary to permit FE from a flat ZnO film (
Figure S2a), which is far above the electrical breakdown strength of a real FE measurement setup. Nevertheless, a field enhancement factor of γ = 58, as derived from the measurements of ZnO nanowhiskers on n-doped Si, enables the emission of electrons at a reasonable applied electric field of less than 35 V/µm (
Figure S2b) [
48,
49].
Several models have been established for the theoretical derivation of the field enhancement factor from the emitter’s geometry [
50,
51]. The so-called “hemi-ellipsoid on a plane” model can be used to calculate γ for a protrusion having an apex width which is much smaller than the radius of the emitters’ base, as it can be observed for the investigated ZnO nanowhiskers (
Figure 8). Specifically, based on determined values of the nanowhisker’s geometry by SEM analysis, theoretical field enhancement factors between 180 and 400 can be calculated for ZnO nanowhiskers grown on bulk substrates being a factor three to eight-fold larger than the extracted γ values from the data fit. However, most models only consider single, free-standing emitters and do not take into account the suppression of the field enhancement effect by electrical field screening arising from neighboring emitters [
52,
53]. Such a screening effect is rather likely in our case because the distance between neighboring nanowhiskers is rather small (high density) leading to a significant field screening and thus, may explain the deviation of the calculated γ and the field enhancement values, which were derived from the linearized experimental data. As a consequence, one can assume that a less densely packed array of the same ZnO nanowhiskers might have a lower threshold field [
54].
The observed small difference in threshold field by about 3 V/µm between ZnO-nanowhiskers on n-doped Si and Ti substrate is in agreement with the field enhancement factors derived from the experimental field emission data by applying the FN model. The aspect ratio of 14 ± 5 for ZnO nanowhiskers grown on n-doped Si is similar to whiskers on Ti with a length-to-diameter ratio of 11 ± 7. Further, the average angle distribution of the nanowhisker tips grown on Ti is only slightly broader with 37 ± 6° than on n-doped Si with 32 ± 4° pointing to an only slightly larger radius of curvature at the emitters’ tip on Ti substrate. Based on the theoretical models, both the aspect ratio as well as the tip curvature should result in a similar enhancement effect. Note, the field emission model by Fowler and Nordheim considers only metal emitters; for semiconducting materials, effects such as electric field penetration, band bending or Schottky barriers at the substrate-electrode interface can alter the electron emission properties and thus, can influence the turn-on field [
48].
In literature, turn-on fields for FE from ZnO nanopillars grown without any catalyst vary from about 22.8 V/µm [
31] down to 5.3 V/µm [
35], depending on the growth setup and substrate type used, but have in common that Zn powder was utilized as precursor [
36,
37]. The turn-on field is reported to be below 10 V/µm for nanowhiskers grown by vapor-liquid-solid dominated processes because the usage of a catalyst can cause a higher aspect ratio [
55] and additionally, offers control about the nanowhisker density, which is essential to reduce the field screening effect [
33]. However, the influence of the metal catalyst at the emitters’ tip is often not considered in the analysis of the field emission data.
Finally, ZnO nanowhiskers grown on a Ti-coated, freestanding SiN membrane showed FE at higher turn-on fields than nanowhiskers on bulk substrates. It was expected from earlier reports that the displacement of the membrane under the applied electrostatic field could cause an additional enhancement of the FE [
56], however, the contrary effect was observed. On the one hand, the large membrane thickness of 1.5 µm might hinder the bending of the membrane and therefore, suppress any additional enhancement effect. On the other hand, the reduced nanowhisker length on the free-standing silicon nitride membrane might be related to slightly different local temperatures during deposition caused by the low thermal contact between membrane and ceramic boat. As a consequence, a smaller aspect ratio of the nanopillars (8 ± 5) compared to ZnO nanowhiskers grown on bulk substrates is observed. This reduced aspect ratio leads to a smaller field enhancement factor—mirrored in the fitted values (
Figure 11b)—which in turn leads to a larger threshold field for electron emission.
5. Conclusions
The effect of the process parameters, such as growth temperature, substrate position and size, gas flow, used amount of zinc precursor, growth time and substrate type on the morphology of ZnO nanowhiskers grown by low-temperature vapor-solid mechanism utilizing zinc acetylacetonate hydrate as a zinc precursor was investigated. The parameters were optimized to receive nanowhiskers with the nearest distribution in dimensions and aspect ratios as high as possible for using them for electron field emission applications.
We showed for the first time, FE measurements from ZnO nanowhiskers grown with the precursors zinc acetylacetonate hydrate and oxygen. Modifications of the turn-on field for FE were observed in dependence on the utilized substrate type (bulk vs. membrane), which were explained by the structural differences of the nanowhiskers, affecting the geometrical field enhancement effect.
Expansion of the accessible experimental parameter range during growth or an additional post-growth treatment, such as thermal annealing or coating by atomic layer deposition, to name a few of them, may give the experimentalist the opportunity to tune the electronic properties of the emitter surface, which could enhance field emission at lower electric fields [
57,
58,
59].
Moreover, nanowhisker growth on thin membranes could potentially be applied in the recently developed nanomembrane detector for MALDI-TOF mass spectrometry, which is a field emission based detection system that has the capability of extending the accessible mass range for a time-of-flight mass spectrometer [
56].